It is well known that bicycles are designed to be driven by the leg power of a rider. The rider actuates a pedal system to transmit motion to the rear wheel of the bicycle by way of a chain drive. In many cases, pedal power is augmented by a transmission that alters the force required on the pedals by shifting the chain drive to sprockets of different size associated with the rear wheel of the bicycle. The sprockets are arranged in a cassette and act to provide the chain drive with different pinion ratios. Some bicycles include front or rear suspensions that provide springs or shock absorbers between the bicycle seat and the wheel, to absorb bumps and dips by allowing the wheel to move with respect to the bicycle seat.
Auxiliary power drives are known using electric or internal combustion motors to apply power to either the front or rear wheel of a bicycle. These devices often use friction drives to engage the bicycle wheel to provide forward motion of the bicycle without pedaling. Such devices have been proposed for attachment to conventional bicycles. However, prior friction drive units are typically rather large and bulky, occupying significant space around or near the bicycle frame, and offsetting the center of gravity and balance of the bicycle. These drive units are often difficult to engage or disengage from the bicycle. Mounting the unit to the bicycle requires significant time and the use of external tools. Upon reaching a destination, the drive unit cannot be easily removed to store the bicycle. Some drive units are permanently attached to the bicycle frame, limiting the flexibility of use of the bicycle as well as the drive unit. Additionally, these drive units often cannot be used on a bicycle with a rear suspension, as the drive unit components and mounting equipment cannot accommodate movement of the rear tire relative to the bicycle frame. Also, some prior drive units accelerate wear of the bicycle tire, beyond the normal wear caused by the road or ground surface.
Therefore, there remains a need for a drive unit for a bicycle that is easier to engage and disengage from the bicycle, and that can accommodate a bicycle with a rear suspension, without causing excessive tire wear.